Introduction: The lumbosacral junction is a susceptible transition point between the lumbar spine and pelvis. Numerous techniques are employed to establish solid lumbosacral fixation and arthrodesis, including iliac screws, posterolateral fusion, and interbody fusion. However, as construct length increases there are increasing forces exerted at the terminal L5/S1 interspace. Presently there is little biomechanical data correlating fixation methods at L5/S1 to thoracolumbosacral construct length.

Methods: By using a robot capable of motion in six axes, force-moment sensor, motion-tracking camera system and software, we simulated the spinal loading effects in flexion-extension, axial rotation, and lateral bending, and compared torques in different construct groups of T4-S1, T10-S1, and L2-S1. L5-S1 and sacroiliac ROM were determined in each primary loading axis for each surgical condition and simulated construct length. Comparisons were made between different surgical conditions, and comparisons were made between various construct lengths. By conducting multidirectional flexibility testing we assessed the effects of constructs of various lengths on the L5/S1 segment.

Results: Under physiological loads: 1) long LS constructs (L2-S1) may be equivalently stabilized by L5/S1 ALIF alone, without iliac screws; 2) longer TLS constructs began exerting increasing motion at L5/S1, exhibiting trends in favor of ISF when extending to T10 and statistically improved fixation when extending to T4; 3) TLS constructs with iliac screw fixation exhibited a statistical benefit from the addition of ALIF support when extending T4-pelvis but not T10-pelvis.

Conclusions: Our findings show that, when considering the torques imposed by long TLS constructs (T4-Pelvis), ISF statistically reduces L5/S1 LB ROM by 93% as compared to native and by 29% as compared to ALIF alone. This confirms that iliac screws are the most effective in supporting LS fixation and protecting against instrumentation failure. Based on this biomechanical model, we find that although not equivalent at higher loads, ALIF alone may sufficiently support L2-S1 constructs, reducing L5/S1 ROM and transmitting loads instead to the sacropelvis. Furthermore, ALIF was found to add significant stability to the long TLS construct (T4-Pelvis) when added to ISF. This difference was not significant for short TLS constructs (T10-Pelvis).

Patient Care: We feel strongly that the results of this manuscript can be useful to the journal readership given the lack of definitive quantification of the biomechanical effect of various surgical constructs.

Learning Objectives: By the conclusion of this session, participants should be able to: 1) demonstrate that ALIF alone may sufficiently support L2-S1 constructs 2) demonstrate that ALIF adds significant stability to the long TLS construct (T4-Pelvis) when added to iliac screw fixation. 3) assess the effects of constructs of various lengths on the L5/S1 segment

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